Thermoplastic polyurethanes (TPU) are of great industrial importance because of their good elastomer properties and thermoplastic processability. An overview of the preparation, properties and uses of TPU is given e.g. in Kunststoff Handbuch [G. Becker, D. Braun], volume 7 “Polyurethane”, Munich, Vienna, Carl Hanser Verlag, 1983.
TPU are usually built up from linear polyols (macrodiols), such as polyester, polyether or polycarbonate diols, organic diisocyanates and short-chain, usually difunctional alcohols (chain lengtheners). They can be prepared continuously or discontinuously. The best-known preparation processes are the belt process (GB-A 1 057 018) and the extruder process (DE-A 19 64 834).
Thermoplastically processable polyurethane elastomers can be built up either stepwise (prepolymer metering process) or by simultaneous reaction of all the components in one stage (one-shot metering process).
A disadvantage of TPU is their easy flammability. To reduce this disadvantage, flameproofing agents, such as, for example, halogen-containing compounds, are incorporated into the TPU. However, the addition of these products often has an adverse effect on the mechanical properties of the TPU molding compositions obtained. Halogen-free self-extinguishing TPU molding compositions are also worth aiming for because of the corrosive action of the halogen-containing substances.
Above all, if high requirements in terms of mechanical properties are imposed it is worth aiming for the use of flameproofing agents which are capable of being incorporated. Such agents are described, inter alia, in U.S. Pat. No. 7,160,974 B and DE-B 102 38 112. In these, a flameproofing agent based on phosphonates or phosphine oxides which are capable of being incorporated is employed in a multi-stage process. TPU having mediocre properties are obtained.